I found this website while answering a reference question. The patron needed a manual for a used sewing machine she had just bought. Unfortunately, I wasn’t able to find one for free online or in WorldCat; I could only find them for sale — about $10, various places.
SewUsa was one of those places. Even though they sell the complete manuals, they also offer the threading diagrams for free. So at least the patron could get started with her new machine.